Mechanical powers have been used to study energy flows in human movement. By indicating whether muscles at a joint are used to generate or absorb power, these parameters can identify the function of joint activity during gait. Our immediate objective is to use joint powers (rotational) and segmental powers (translational) to evaluate changes in ankle function before and after forefoot arthroplasty for rheumatoid arthritis. Our goal is to develop an objective measure of improved push- off capability following surgery. We plan to analyze patient gait pre- operatively, and at six and twelve months post-operatively. Energy flows into the foot will indicate non-propulsive lifting of that same segment, while energy flows out of the foot will indicate the use of the foot to push the body forward. Lower extremity kinematics are collected using a passive infra-red motion analysis system. Ground reaction forces (GRF) are collected using two strain gauge force plates. Inverse dynamics based upon rigid body assumptions are used to provide resultant forces and moments at the ankle (early protocol) and at the knee and hip (amended protocol). Early methodological studies found that joint power curves during stance in walking typically showed high frequency components. This was curious since the joint moment and angular velocity data used to calculate powers showed little or no high frequencies. We found that digital low- pass filtering of GRF, still preserving 99% of the signal content, attenuated the high frequency peaks in joint powers. As more patients are studied at the ankle, knee, and hip, joint powers may be used to show functional changes among the lower extremity joints. However, since only a few patients have pre- and post-operative data available for all three joints of the lower extremity, continued joint power analyses are not feasible at this time. We are shifting our emphasis away from joint powers and toward segmental powers at the proximal end of the foot to determine energy flow patterns.